Trump said he let countries know during this week's summit that he was "extremely unhappy". "After we're at 2 percent, we'll start talking about going higher".
"It will be over a relatively short period of years", Trump said.
Trump repeated demands on the second day of the summit for countries to meet a spending target of two percent of GDP now, instead of by 2024, and to eventually double spending to four percent. And he claimed the other 28 members had agreed to boost their own defense spending to as much as 4 percent of gross domestic product - a claim that other leaders disputed.
Donald Trump has once again proclaimed himself to be a "very stable genius".
"Sometimes if I didn't know better, I might say that we are purposefully trying to destabilize the Western alliance and turn the world upside down", he said.
"Everybody in the room thanked me", he said, "There's a great collegial spirit in that room that I don't think they've had in many years".
Trump told the tabloid, Britain's best-selling newspaper, that he would have gone about Brexit in a different way.
It came while defending himself against allegations he was unfit to be United States president following publication of the book, Fire And Fury: Inside The Trump White House. "It confirms the goal of 2 percent by 2024".
Following the summit, NATO Secretary General Jens Stoltenberg was careful not to contradict the president's claim of a substantial increase in spending but was only able to directly point to members' existing 2 percent commitments.
Former NATO supreme allied commander James Stavridis tells Power & Politics that Canada is punching above its weight on NATO operations, but could do more. And the president's declaration was also directly contradicted by one of his closest personal allies, French President Emanuel Macron.
At NATO's 2014 summit in Wales, in the face of new Russian aggression and the annexation of Crimea, the allies had pledged to increase investments in defence and move towards investing 2 per cent of their GDP in collective security. In response, a North Atlantic Treaty Organisation official said Tbilisi would join the alliance after meeting all of the necessary requirements, stressing that the members of the alliance should first reach a consensus on a Membership Action Plan for the country.
He also vowed to raise allegations of Russian meddling in the 2016 USA presidential election, amid an ongoing U.S. investigation into whether his campaign colluded with Moscow.
"The final declaration has been signed by all 29 members of the alliance, including the USA", the French minister noted. It will be Trump's first working visit to the United Kingdom since he became President, and massive protests in London are expected.
"I am very glad that we are united today in freedom as the Federal Republic of Germany and that we can therefore also make our own independent policies and make our own independent decisions".
The Metro carries a picture of the two leaders together, with the main story on Donald Trump's views that many Britons agree with him.